(a) Field of the Invention:
The present invention relates to an ocean environment monitoring system and a method for controlling the same. More specifically, the present invention relates to a technique for preventing settlement of marine organisms on sensors used in an ocean environment monitoring system.
(b) Description of the Related Art:
After sensors for monitoring ocean environmental factors such as an electric conductivity of seawater, dissolved oxygen, etc., are located in seawater for a prolonged period of time, errors in measurements are generated due to dirt formed by the settlement of marine organisms (marine biofouling or biofouling). To prevent marine organisms from adhering or settling onto sensors disposed in direct contact with seawater, a method has been proposed in which the sensors are received in a chamber having an inlet port and an outlet port for seawater. The interior of the chamber and filters attached to the ports are made from a material producing copper ions therefrom.
A technique is known in which a direct current is supplied in an equipment of an on-shore plant which uses seawater or a structure or vessel used in oceans. The current flows through the seawater to electrolyze the seawater to produce an antifouling substance such as chlorine and hypochlorous acid, thereby preventing marine biofouling. Unlike copper ions, such an antifouling substance does not accumulate in the sea because it is decomposed by other substances in seawater, sunlight, and the like.
Examples of the method for preventing settlement of marine organisms onto the above structures utilizing electrolysis of seawater are described in Patent Publication Nos. JP-A-3-125690, JP-A-4-148313 and JP-B-58(1983)-103345. In the method described in JP-A-3-125690, current is supplied to flow through a conductive coating film coating an object to be protected, thereby producing an antifouling substance at the surface of the object. In the method described in JP-A-4-148313, seawater is electrolyzed in an electrolysis vessel to produce an antifouling substance, the antifouling substance being included in seawater passing in the vicinity of an object to be protected.
In preventing settlement of marine organisms onto sensors used for monitoring an ocean environment, it is desired that such protection of the sensors can be made at a low cost without causing environmental contamination and that the monitoring system including such a protection can be easily maintained. However, the conventional method for producing copper ions in the sensor chamber only provides insufficient effects, and filters disposed at the inlet port and outlet port for seawater must be exchanged at least each month or every two months because these filters become clogged by settlement of marine organisms and the like. In addition, since copper ions, which are not considered entirely harmless, are continuously dissolved into seawater, a risk in environmental contamination exists.
The method in which marine biofouling is prevented by using an antifouling substance, which is produced by electrolysis of seawater by using direct current, has been applied to some structures used in oceans. However, the method has never been applied to sensors used in an ocean environment monitoring system. If the method, in which an antifouling substance is produced on the surface of an object to be protected, is to be applied to the sensors used in an ocean environment monitoring system, it is considered that there is a large possibility that the function of the sensors themselves deteriorates due to the conductive coating film or current flowing therethrough.
In the method in which seawater is electrolyzed in an electrolysis vessel to generate an antifouling substance included in seawater passing in the vicinity of an object to be protected, the antifouling substance is supplied into a complete open system so that it is dispersed by waves or tidal current. Hence, the antifouling substance must be produced continuously to maintain the concentration of the substance at a constant level in the vicinity of the object to be protected. In this case, a huge amount of electric power is consumed in the monitoring system. Also, there are problems that the equipment for electrolysis must be large, and the electrodes deteriorate due to damages or chemical erosion. Similar problems will arise even if electrodes are disposed in the vicinity of sensors without providing an electrolysis vessel. Moreover, in some items of measurement, data obtained by sensors contain significant errors if the antifouling substance exists around the sensors. Therefore, it is generally considered that the method should not be applied to sensors used for such measurements.
In an ocean environment monitoring system in which sensors are disposed to perform long term monitoring and they are located away from a structure or ship in the ocean, the overall size of the system and the amount of usable electrical power are limited. Accordingly, reduction of the size of the system, reduction of power consumption, and no maintenance work over a prolonged period of time are desired.